Learn Spanish (Latin America): Ese hombre es leal; no lo traicionen.

This is a negative command. In Spanish, object pronouns like lo must go before the verb in negative commands, right after no. If this were a positive command (betray him), the pronoun would attach to the end of the verb: traiciónenlo.

Who is being told not to betray him?

The verb traicionen ends in -en, which is the command form for ustedes (you all). The speaker is addressing a group of people. If they were talking to just one person informally, it would be no lo traiciones.

Why use lo instead of le for him?

The man is the direct object of the verb because he is the one directly receiving the action of being betrayed. Therefore, we use the direct object pronoun lo (him). We would use le if he were the indirect object, like if we were giving a gift to him.

Why is it es leal instead of está leal?

We use es (from the verb ser) because being loyal (leal) is viewed as a core personality trait or characteristic of the man, rather than a temporary state or emotion.